1. Field of the Invention
This invention pertains generally to measurement of analyte in gas-borne particles, and particularly to collection of gas-borne particles into a liquid so that analytes in the particles can be analyzed.
2. Description of the Related Art
Devices are needed that can quickly detect and identify the presence of harmful materials in airborne particles. Airborne infectious agents such as bacteria and viruses can transmit diseases of humans, other animals, and plants. Some of these infectious agents, as well as some protein toxins have been used as biological-warfare (BW) agents. Some airborne proteins and pollens cause allergies. Also, improved methods for characterizing aerosols would be useful for understanding atmospheric chemistry, including the sources, chemical reactions, and fates of atmospheric particles. Here, “airborne particle” or “gas-borne particle” refers to both the solid particles and liquid droplets in an air or gas sample.
A group of particles in an air sample, of either indoor or outdoor air, may include many types of organic and inorganic materials and many types of bacterial and other biological materials. Individual particles may be homogenous or may be a complex mixture of materials. Identification of specific bacteria, viruses, or proteins, and many other types of complex chemicals that may be mixed in atmospheric particles requires first the collection of those particles so that they can be analyzed by appropriate techniques, e.g., immunoassay for bacteria, protein allergens, and other biological materials; culturing for bacteria and viruses; mass spectrometry for organic chemicals. For identification of specific bacteria or proteins using biochemical methods, the collection of these particles into a liquid is needed. For many types of analyses, e.g., single-particle infrared or Raman analysis, collection of airborne particles onto filters or other solid surfaces is desired, but that is not the interest in this invention.
One reason that there is a need for improved aerosol-into-liquid collectors is that there is a need for improved instruments which detect harmful biological aerosols, and  which: (i) are sensitive to small numbers of particles in the air; (ii) are specific for whatever specific biomaterials the user desires to detect; (iii) provide a rapid response, with no more than a short delay between the time the analyte aerosol enters the instrument and the time the instrument indicates that an analyte has been identified; (iv) can run continuously; (v) has a low requirement for consumables; and (vi) has little need for operator time. If an aerosol into liquid collector, as part of an instrument to detect harmful aerosols, can collect the particles from a large volume of gas such as air into a small volume of liquid, then the overall instrument can be more sensitive and have a more rapid response, and if this instrument also does not loose a large amount of the collection liquid during the collection of the particles, it can have a lower requirement for consumables.
Investigators have worked for years to develop instruments for collecting aerosol particles into liquids. The literature describing different methods is large. Methods for collection of airborne particles, including methods for collection of particles into liquids, have been reviewed by M. L. Muilenberg, “Sampling Devices,” Immunology and Allergy Clinics of North America, 23, 337–355 (2003). Liquid-impingement methods are widely used, but the volumes of liquid needed for microfluidic analyses are typically far smaller than are used with liquid impingers.
Some methods of collecting particles into a liquid are briefly summarized in the paper by I. Belostotski, V. V. Gridin, I. Schecter, and C. N. Yarnitzky, “Combined micro-droplet and thin-film-assisted pre-concentration of lead traces for on-line monitoring using anodic striipping voltammetry,” Analytical Bioanalytical Chemistry, 375, 482–488 (2003), especially in the introduction of that paper.
S. Zaromb, “High-Throughput Liquid-Absorption Air-Sampling Apparatus and Methods,” U.S. Pat. No. 6,087,183, describes an apparatus to collect particles from air into liquid. In his apparatus 200 to 300 liters/min of air are drawing asymmetrically into a cylinder. This rapid flow of air swirling in the cylinder causes the liquid at the bottom of the cylinder to rise and also swirl inside the cylinder. Then particles from the air are collected into the liquid. The apparatus uses at least a few ml liquid per sample, and evaporation may be expected to be significant.
P. T. Call, V. M. Kenning, C. Call, J. G. Birmingham, and D. J. Hammerstrom, “Impact Particulate Collector Using a Rotary Impeller for Collecting Particulates and Moving a Fluid,” U.S. Pat. No. 6,267,016 B1 (2001), describe another means to collect particles into a liquid. The airborne particles impact upon a rotating surface. A liquid is injected into the cavity that contains this rotating surface, and the particles may be washed into this liquid or may impact directly into the liquid.
P. J. Coyle, T. A. Pletcher, T. J. Davis, and S. Mangru, “Method and Apparatus for Concentrated Airborne Particle Collection,” US Patent Application Publication, 2004/006907 A1, herein incorporated by reference, describe an apparatus to collect particles from a large volume of air into a relatively small volume of liquid in which a hydrophobic membrane establishes a “controllable air-fluid boundary” between a separation section where the particles are in air, and a capture section in which the particles are transported in liquid. A corona charger may be used to charge the aerosol so that the particles may be focused into the liquid stream.
I. A. Agranovski, V. Agranovski, S. A. Grinsphun, T. Reponen, and K. Willeke, “Collection of Airborne Microorganisms into Liquid by Bubbling through Porous Medium,” Aerosol Science and Technology, 36, 502–509 (2002), describe a means to collect aerosols by bubbling them through a fibrous filter that is immersed in water.  They demonstrate very efficient collection for an air sample of up to 4 liters/minute running for up to 8 hours.
D. A. Masquelier, F. P. Milanovich and K. Willeke, “High Air Volume to Low Liquid Volume Aerosol Collector,” U.S. Pat. No. 6,520,034 B1, herein incorporated by reference, provide an apparatus to efficiently collect particles from a high volume (e.g., 225 Lpm) of air into a “collecting liquid having a volume not greater than about 100 to 300 microliters.” They provide a means to reduce the loss of the collection liquid, which they need to reduce because they impinge the 225 liters/min of air onto this liquid in order for the particles in the air to be transferred to the liquid. This impinging of air into the liquid causes some of the liquid to form airborne droplets and some of the liquid to evaporate. They collect for re-use the liquid droplets, but not the evaporated liquid. The device is, small (typically 2 liters), lightweight (typically less than 1 Kg), and has low power requirements. The typical volume of liquid required per day is not stated.
V. V. Gridin, I. Litani-Barzilai, M. Kadosh, and I. Schecter, “A Renewable Liquid Droplet Method for On-Line Pollution Analysis by Multi-Photon Ionization,” Analytical Chemistry, 69, 2098–2102 (1997), herein incorporated by reference, describe collection of gasses and aerosol particles into a pendant water droplet, where they are analyzed. The collection efficiency is not expected to be large, but would be sufficient for cases where particles that are in high concentration, and/or when the collection times are long.
V. Gartstein and A. D. Willey, “Dynamic Electrostatic Aerosol Collection Apparatus for Collection and Sampling Airborne Particulate Matter,” US Patent Application Publication, 2004/0089156 A1, published May 13, 2004, describe a method of collection of aerosol particles by passing them through a spray of charged droplets that act as electrostatic collectors. The particles may be given a charge opposite to that of the droplets to enhance the collection efficiency. The liquid is re-circulated and is chosen to have a low volatility to reduce evaporative losses. The invention has similarities J. B. Fenn's, “Electrospray Air Sampler,” US Patent Application Publication 2004/0023411 A1, published Feb. 5, 2004.
S. C. Hill, Aerosol Particle Analyzer for Measuring the Amount of Analyte in Airborne Particles, U.S. patent application Ser. No. 10/708,191, filed Feb. 14, 2004 and herein incorporated by reference, described a method for collecting particles into levitated droplets of an analysis liquid in order to rapidly detect analytes in the particles, and stated that the droplets of the analysis liquid could be collected and further analyzed. The use of a linear quadrupole to focus both the airborne particles and the airborne droplets of the analysis liquid to the axis of the linear quadrupole in order to increase the probability of collisions between particles and droplets is also described in that application. That invention was aimed specifically at the detection of analytes in particles without having the particles come into contact with any surface, so that there would be no problem of cross contamination from sample to sample. However, for detection of analytes or particles in cases where the tubing and instrumentation can be constructed so that the particles and analytes do not stick or cause cross contamination, the method is more complicated than required.
H. B. Lin and S. C. Hill, Aerosol Particle Analyzer for Measuring an Analyte in Airborne Particles, U.S. patent application Ser. No. 10/816579, filed Mar. 26, 2004, and herein incorporated by reference, described an apparatus for collection of particles into a small volume of an analysis liquid at the end of a capillary tube, where the rate of the collection of the particles is enhanced by electrostatic forces between the particles. The particles are given a charge that is opposite to that of the collection liquid at the end of the capillary tube. The goal was to measure the amount of an analyte in the particles, during the time shortly after the particles collide with and mix with the analysis liquid. However, it is stated that droplets of the analysis liquid with the combined particles could be stored for further analysis.
In summary, key prior art regarding collection of aerosol into a small volume of liquid provides for:                a) collection of particles into a flowing liquid that can be drawn directly into an analyzer, where aerodynamic forces are used to cause the particles to impact into the liquid (Gridin); because the gas moves slowly enough that it does not disrupt the slowly flowing liquid, the sampling rate and collection efficiency are relatively low;        b) collection of particles into a small volume of liquid using impaction of particles from a rapidly flowing gas (Willeke); because the gas flows rapidly, the sampling rate and collection efficiency can be large, the evaporation of the liquid into which the particles are collected can be relatively high, and the flow of the liquid is disrupted by the airflow;        c) collection of aerosol using a combination of virtual impaction, impaction, and electrostatic forces, and using a hydrophobic membrane (Coyle);        d) collection of particles using a spray of charged droplets (Fenn, Gartstein); if the liquid has a very low volatility to reduce the loss liquid, e.g., polyethers, as described by Gartstein, the particles would probably need to be removed from this liquid and responded in a aqueous solution before an antibody- or aptamer-based analysis could be accomplished;        e) collection of particles into a small droplet of a liquid that is either levitated or flowing in an air flow in a quadrupole (Hill, U.S. patent application Ser. No. 10/708,191).        f) collection of particles into a small volume of an collection liquid at the end of a capillary tube, where electrostatic forces provide the means to collect particles from the air without using such a high air flow that the liquid is disrupted and the liquid evaporates rapidly.        